1. Field of the Invention
The present invention generally relates to electronic systems, and more specifically to systems using electromagnetic transponders, that is, transceivers capable of being interrogated in a contactless and wireless manner by a read and/or write terminal.
2. Discussion of the Related Art
Many communication systems are based on a modulation of an electromagnetic field generated by a terminal. They range from a simple electronic tag used as a theft-prevention device to more complex systems where a transponder capable of communicating with the terminal when present in the field thereof is equipped with calculation functions (electronic purse, for example) or data processing functions.
Electromagnetic transponder systems are based on the use of oscillating circuits comprising a winding forming an antenna, on the transponder side and on the terminal side. Such circuits are adapted to be coupled by near magnetic field when the transponder enters the field of the terminal. The oscillating circuits of the terminal and of the transponder are generally tuned to the same frequency corresponding to the excitation frequency of the oscillating circuit of the terminal.
In most cases, transponders have no autonomous power supply and extract the power supply necessary to their circuits from the high-frequency field radiated by the antenna of the terminal.
The quality of the communication and of the possible power transfer depends on the coupling between the terminal and the transponder. This coupling, which is inversely proportional (non linear) to the distance between the terminal and the transponder, conditions the amplitude of the voltage recovered by the transponder. It is thus desirable to be able to evaluate the current coupling factor between a transponder and a terminal having it in its field.